Up to now, various optical deflection apparatuses in which a mirror is resonated have been proposed. As compared with a light-scanning optical system using a rotary polygonal mirror such as a polygon mirror, a resonance type optical deflection apparatus has the features that the size of the optical deflection apparatus can be significantly reduced; power consumption is small because a Q value is high; there is theoretically no optical facet angle error; an optical deflection apparatus made of single-crystalline Si manufactured by particularly a semiconductor process theoretically has no metal fatigue and excellent durability (see Japanese Patent Application Laid-Open No. S57-008520).
However, a resonance type optical deflector has a problem in which a material characteristic changes with a variation in ambient temperature to shift a resonance frequency, thereby dramatically reducing a deflection angle. In order to solve the problem, there have been proposed many methods capable of changing a drive frequency in response to a shift of the resonance frequency, including a method capable of changing a drive frequency of an excitation current applied in response to the shift of the resonance frequency by detecting the deflection angle based on induced electromotive force (see Japanese Patent Application Laid-Open No. 2001-305471).
There have been also proposed many methods capable of changing the resonance frequency based on temperature compensation, including a method of controlling the resonance frequency of an optical deflector by using a heater, a temperature sensor, and a temperature control circuit, which are provided for the optical deflector (Japanese Patent Application Laid-Open No. H05-2602 67).
Here, the temperature of the optical deflector is varied not only by ambient temperature but also by laser light modulated according to image data when laser light from a laser light source is deflected, As compared with the former variation by ambient temperature, the latter variation by the laser light is a very rapid change and its variation time can be estimated to be several tens of milliseconds or less. Here, in the case of a low-power laser light source, a variation in temperature thereof is negligibly small. However, when a high-power laser light source is used, a variation in temperature thereof is more significantly caused. In an image-forming apparatus using such an optical deflector, the resonance frequency of the optical deflector changes with a variation in temperature thereof, so that a projected image deteriorates.
A time required to stabilize a variation in oscillation state of the optical deflector is proportional to a Q value. Therefore, when the resonance type optical deflector has a high Q value, a stabilization time can be estimated to be several tens of milliseconds. Even when the temperature of the optical deflector is changed by a heater or the like, a stabilization time can be estimated to be several tens of milliseconds. As described above, a variation in temperature of the optical deflector due to the modulation of deflected light is caused on the same order as an oscillation stabilization time of the optical deflector. Thus, it is difficult to control the variation in temperature of the optical deflector due to the modulation of the deflected light using the above-mentioned conventional techniques.